1. Technical Field
The present invention relates to a cooling duct that directly injects emergency core cooling water which is supplied from a high-pressure safety injection pump or a safety injection tank, to a downcomer, which is formed by a reactor pressure vessel and a core barrel in a pressurized light-water nuclear reactor. In particular, the present invention relates to a longitudinally divided emergency core cooling (ECC) duct for an emergency core cooling water injection that includes side supports for the cooling duct, has structural stability while thermally expanding and contracting, and is divided into a plurality of longitudinally-divided ducts in the longitudinal direction of the cooling duct.
2. Related Art
A pressurized light-water nuclear reactor is designed with consideration for a sufficient safety factor, but accidents could happen contrary to expectations. If the emergency core cooling water is not sufficiently supplied to the nuclear reactor during an accident (for example, the leakage of a large amount of cooling water), a core of the nuclear reactor could get overheated so that the nuclear reactor may get damaged. The pressurized light-water nuclear reactor is provided with the high-pressure safety injection pumps and safety injection tanks so that the emergency core cooling water is supplied to the nuclear reactor from the outside to cool the core when the accident (for example, a loss of coolant) occurs. The methods of supplying emergency core cooling water are classified into a cold leg-injection method that uses an injection nozzle positioned at a cold leg and a direct injection method that uses an injection nozzle positioned at a reactor pressure vessel.
The cold leg-injection method has a problem in that all emergency core cooling water leaks through a fractured portion when the cold leg is fractured, which causes the effectiveness of the cooling of the core of the nuclear reactor to deteriorate. In order to solve the problem of the cold leg-injection method, there has been employed an improved injection method that provides a direct vessel injection (DVI) nozzle for directly supplying the emergency core cooling water to a downcomer between the reactor pressure vessel and a core barrel by using a direct vessel injection nozzle, thereby preventing the emergency core cooling water through a cold leg.
However, the direct injection method also has a problem in that emergency core cooling water is directly discharged to the outside of the reactor pressure vessel due to the strong cross-flow in the downcomer, which causes a flow of the injected emergency cooling water toward the broken cold leg bypassing the core. In order to solve this problem, there has been proposed a method disclosed in U.S. Pat. No. 5,377,242 (James D. Carlton et. al) where a direct vessel injection nozzle merely extends to a core inlet: a method disclosed in U.S. Pat. No. 5,135,708 (James D. Carlton et al.), and Korean Patent Application publication No. 2000-0074521 (Hanlim. Choi et al.) where a narrow gap is formed between a direct vessel injection nozzle and a cooling duct and a cooling duct merely extends toward the lower side of a downcomer. In these cases, when a pipe of the direct vessel injection nozzle is fractured, the lowermost end outlet of the cooling duct is reversed as an inlet for fracture flow so that the level of the cooling water in the nuclear reactor is constantly lowered and reaches the lowermost end outlet of the cooling duct or below. If the level of the cooling water is lowered as described above, the core of the nuclear reactor is exposed, which may result in fatalities.
According to a technique concerning a pipe, which injects emergency core cooling water, of “safety injection system including extension duct for core barrel (Taesoon. Kwon et al., Korean Patent Application No. 2008-0024306)”, since the bearing resistance, which is born against strong cross-flow in a downcomer by a cooling duct, is weak, the pipe is structurally weak. Further, when a nuclear reactor starts at a low temperature and is heated to a high-temperature output condition, the pipe is weakened against a thermal stress at the welded portions of the structures. In particular, as the length of the cooling duct is increased, the pipe becomes structurally weaker against the thermal stress that is caused by thermal expansion and contraction.
There is a demand for an emergency core cooling (ECC) duct for emergency core cooling water injection that can prevent reversal of water levels at the inlet and outlet when an accident (for example, the fracture of a direct injection pipe) occurs, has sufficient structural durability to bear a strong cross-flow in a downcomer, and suppresses the thermal stress that is generated at a welded portion of an injection duct until a nuclear reactor reaches a high temperature after starting at low temperature, and can prevent an excessive load of backflow that is applied to the ECC duct during an initial core backflow when a large cold leg is fractured.